Dual-lumen catheters have come into widespread use for extracorporeal blood purification procedures such as hemodialysis. Blood is withdrawn from the patient through one of the lumens of the catheter and supplied to a hemodialysis unit where the blood is dialyzed. The resulting dialyzed blood is then returned to the patient through the other lumen of the catheter. Examples of such catheters are shown in U.S. Pat. Nos. 4,134,402; 4,583,968; 4,568,329 and 4,692,141.
Many catheters for relatively long-term use are made of either polyurethane or silicone. Many polyurethane catheters are sufficiently rigid to be introduced into a patient's vein percutaneously without surgery. However, one of the difficulties with many polyurethane catheters is that they may be incompatible with the human body when left in the patient for relatively long periods of time such as a month or more.
The silicone catheters may be left in place for longer periods of time than many of the polyurethane catheters without allergic reactions or traumatic problems in most patients. However, one of the difficulties with silicone catheters is that the initial insertion of such catheters usually requires surgical intervention because the soft, pliable, elastic properties of the silicone which contribute to its compatibility with the human body are the same properties that make it difficult or impossible to insert the silicone catheter percutaneously into the vein of the patient.
Another problem encountered in using multiple lumen catheters for hemodialysis results from the flow of fluid through the lumens of the catheter. Typically one of the lumens of the catheter extends the full length of the catheter and is known as the venous or blood return lumen because the blood is returned to the body of the patient therethrough. The other lumen typically extends between the proximal end of the catheter and an opening in the sidewall of the catheter. This lumen is known as the arterial lumen because the blood flows through the lumen away from the body of the patient to the dialysis machine. During use of a hemodialysis catheter, a significant pressure differential is created across the septum due to the simultaneous flow of blood through the venous lumen under a positive pressure and through the arterial lumen under a negative pressure. With certain catheters, such as a silicone catheter, it is possible for the septum and the outside wall of the arterial lumen to collapse together, thereby closing the lumen and restricting the flow of blood therethrough. It is believed that the deflection of the septum is caused, at least in part, by the stretching of the septum or deformation of the lumens by the pressure gradient caused by the opposite flows of fluid through the lumens. This is particularly likely to occur in situations where the catheter is curved or bent.
A further problem which may be encountered when using single or multiple lumen catheters results from the repeated or long-term clamping of the extension members of the catheter. If the extension member is repeatedly clamped or clamped for a long period of time, the sidewalls of the extension member may stick together, and the catheter may become unusable. It is a common practice to use extension members formed of PVC or polyurethane material having a soft or lower durometer of about 72 to 85 A to provide the desired flexibility for the extension member. Although these materials have the desired flexibility, some of the softer materials may also have a tendency to become sticky on repeated or prolonged clamping and may remain partially or fully closed after repeated or prolonged clamping. Some catheters use silicone extension members which have sufficient flexibility for the intended use, and the sidewalls of the extension members do not have a tendency to stick together. The difficulty with the use of silicone materials for extensions is that silicone is extremely difficult to permanently bond to materials other than silicone, and the silicone extensions have a lower tear and tensile strength which may weaken the wall of the extension member after repeated clamping. Other commercially available catheters use a chemical coating on the interior surface of the extension to make the interior surface lubricous. One difficulty with the coated extension members relates to the difficulty of applying the lubricous coating to the extension member, and the coating may also wear off the interior surface of the extension member after repeated flushing of the catheter. Other catheters may use a urethane extension member which incorporates a waxy material therein which blooms to the surface of the extension member. Although the waxy material is believed to reduce the likelihood of the walls of the extension member sticking together, the waxy material may inhibit the adhesion or bonding of the extension member to the remaining components of the catheter.
A further difficulty arises with the use of single or dual lumen catheters when the clinician desires to obtain a blood sample for analysis and/or inject a medication into the blood vessel of a patient. With single or dual lumen catheters, the clinician must disconnect the catheter and/or flush the lumen before and after use. Similarly, when incompatible medications are administered to the patient, the clinician must carefully flush the lumen between medications.
In U.S. Pat. No. 5,221,255 granted to Mahurkar et al. a multiple-lumen catheter is disclosed which comprises an elongated cylindrical tube made of a soft elastic material and having an internal septum extending along the length thereof to form a pair of longitudinal lumens therebetween. A reinforcing member is disclosed as extending along the full length of at least one of the lumens for transmitting forces applied to the proximal end of the tube to the distal end of the tube. In a preferred embodiment of the Mahurkar application, the reinforcing member is embedded in the septum and is made of a material which is substantially stiffer than the material of the tube so that the catheter can be advanced against a resistance by the application of force to the proximal end of the catheter. The reinforcing member is described as also avoiding deformation and/or collapse of one or more of the lumens when a pressure gradient exists across the septum.
In a modified embodiment disclosed in the Mahurkar patent application, the cylindrical catheter has a pair of orthogonal flat internal dividers extending along the interior of the catheter for dividing the interior into three lumens extending along the length of the catheter. One of the lumens disclosed in this embodiment is described as having a D-shaped transverse cross section. The orthogonal dividers form a T-shaped septum which resists kinking of the catheter along the orthogonal transverse planes of the catheter.
As also disclosed in the Mahurkar patent application, the catheter is disclosed as being formed from a soft elastic material such as a silicone. The septum of the catheter is disclosed as being formed of a rigid material such as a nylon strip. Variations of the septum are also disclosed wherein the nylon strip has a T-shaped transverse cross section for a triple lumen catheter, a D-shaped transverse cross section or where the nylon strip is inserted into the catheter and then a thin layer of silicone is formed over the nylon strip to prevent the exposure of the fluids in the catheter to the nylon strip. As a further variation of the disclosed catheter, the dual lumen tube is described as being possibly coextruded with a continuous reinforcing strip of a material such as nylon in the septum.
One of the concerns which may be encountered by the approach disclosed in the Mahurkar patent application is that the nylon strip or other material may separate from the silicone portion of the catheter if the catheter is placed under a tensile load such as by stretching a portion of the catheter prior to insertion. The potential for separation is believed to be due primarily to the suggested use of materials which have significantly different elastic characteristics for the septum and catheter body.
The multiple-lumen catheter disclosed in the Mahurkar patent application also requires a number of additional manufacturing steps which may significantly increase the cost of manufacturing the disclosed catheter while similarly increasing the potential for quality control problems as compared to currently available catheters or the present invention.
Triple lumen catheters such as those described in U.S. Pat. Nos. 3,995,623 granted to Blake; 4,406,656 granted to Hattler et al.; 4,072,146 and 4,894,057 granted to Howes and 5,221,256 granted to Mahurkar, provide three independent dedicated lumens in a single catheter with lumen openings at different sites in the vein. Such catheters permit the simultaneous and continuous infusion and/or monitoring of the patient. As a result of the flexibility provided by these catheters, they are the catheters of choice in managing seriously ill patients. These triple lumen catheters are not optimal for the treatment of critically ill hemodialysis patients because of their inefficient flow geometry. The cross sectional area of these triple lumen catheters is not maximized and therefore the flow characteristics of these catheters is not optimized. In hemodialysis it is desirable to have a catheter which allows a bi-directional flow of approximately 250 ml per minute with a pressure gradient under 100 mm of mercury. Therefore, in order to understand the best way to maximize the geometry of the catheter it is important to understand Poiseuille's Law which relates to the velocity of the flow of a liquid through a constrained area such as a capillary tube or the lumen of a catheter. According to Poiseuille's Law, the velocity of the flow of a liquid though a lumen varies directly to the pressure of the liquid and the fourth power of the diameter of the lumen and inversely to the length of the lumen and the coefficient of viscosity. Therefore, the flow of a fluid through a catheter may be maximized by maximizing the area of the lumen while minimizing the surface area of length of the walls forming the lumen of the catheters. With hemodialysis catheters, this creates a conflict between the need to have two lumens which have the flow characteristics of a dual lumen catheter while providing a third lumen for the administration of medications and/or monitoring of the venous pressure of the patient. In U.S. Pat. No. 5,221,255 granted to Mahurkar, this desire to optimize the size of the first and second lumens is explicitly expressed. Despite this expressed desire, the Mahurkar patent discloses a triple lumen catheter which fails to optimize the cross sectional area of the lumens and which unnecessarily increases the length of the walls surrounding the respective lumens of the catheter. In one of the objects disclosed in the Mahurkar patent, the reason for this compromise in the flow characteristics is described as being necessary to provide a third lumen which may be used to contain a removable stylus to provide the catheter with sufficient column strength to allow for the insertion of the catheter over a flexible Seldinger guide wire without buckling or kinking. As a result of this expressed desire to accommodate a guide wire in the third lumen, the third lumen is shown and described as having a circular cross section which impinges on the cross sectional areas of the first and second lumens. As explained by Poiseuille's Law, this inefficient use of the diameter of the lumens results in a significantly reduced efficiency in all three lumens of the catheter.
Therefore, a need remains for a catheter which maximizes the diameter of the lumens of the catheter and which minimizes the length of the walls which form the lumens.
Similarly, a need remains for a catheter which has sufficient flexibility and softness to be used for prolonged periods of treatment while being able to be inserted nonsurgically and manufactured consistently and cost effectively.